When I was at school in the early eighties, a teacher "lost it" and punched a boy hard. Afterwards the teacher came into my form to tell his head of department and he looked absolutely sick with worry.
The boy was a well-known troublemaker but, to his eternal credit, he admitted that he'd probably pushed things too far and that was the end of it!
Teachers these days are sometimes placed in impossible situations. Kids have all of the rights but none of the responsibilities, and the support network just isn't there for all teachers to avoid all impossible situations at all times. At times when it does break down, you'd hope that your senior management would support you, but many don't have the backbone; as in this case:
Punishment for assaulting a teacher: 4 day suspension
Punishment for mildly reacting to assault: dismissal, end of career
It seems inequitable.